THE IMAGINARY lUNGE AND TROUGH. 37 



to 6,000 feet above the level of the sea ; in the interior of the range 

 is the great plateau of Tibet., which, presenting very considerable 

 irregularities of contour, may, in view of the distance separating 

 it from the stations of observation with which we will be concerned, 

 be regarded as a plain of about 15,000 feet of elevation above the 

 sea level. 1 Along the southern edge of this plateau runs the great 

 snowy range., including the highest peaks, and south of the snowy 

 range comes the region of the lower Himalayas, where the summits 

 do not rise to more than ten or twelve thousand feet above sea 

 level. Though the distinction between these two legions, of the snowy 

 range and the Lower Himalayas, is fairly well defined and somewhat 

 abrupt, the average level of the ground shows a less abrupt change and 

 in the Lower Himalayas themselves there is a gradual decrease in 

 general altitude to about 5,000 feet at the southern margin of the 

 hills, where the ground drops abruptly to the foot-hills, or Sub- 

 Himalayas, of the Siwalik region. These may conveniently be 

 represented in that portion of the range which will come into 

 consideration, by a plateau of twenty miles in width, and fifteen 

 hundred feet in elevation above the plains to the south. The 

 generalised cross-section of the elevated mass of the Himalayas 

 may therefore be represented as a plateau of 15,000 feet in eleva- 

 tion, bordered by an inclined plane of 100 miles in width, sloping 

 from 15,000 to 5,000 feet of elevation, and a plateau of 1,500 feet 

 in height by 20 miles in width. For purposes of calculation, it 

 will be simpler to substitute for this inclined plane a series of steps 

 each ten miles broad and 1,000 feet lower than the next step to the 

 north. The Imaginary Eange would then have a cross-section 

 like that shown in fig. 4 (page 38), where two actual cross-sections 

 of the Himalayan Range are also given, for comparison. 



In the calculations regarding this Imaginary Range, it will 

 be assumed to have an east to west direction, with the elevated 

 plateau on the north and the plains on the south. This is not only 

 in general agreement with the Himalayas, but will allow of deflec- 

 tions towards the range being expressed as northerly deflections, in 

 accordance with the usual convention, by the minus sign, and 



1 I have followed previous writers in accepting 15,000 ft. as the mean height of the 

 central plateau ; actually the mean height would be more correctly 16,000 ft. or a little 

 more. As the elevation of the land south of the Himalayas is ignored in the calculations, 

 and only the height of the hills above the sea considered, the difference is partly eliminated, 

 and in any case would have but a very small effect at the stations at which observations 

 have been made. 



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